1. Field of the Invention
This invention relates to a temperature detecting circuit that generates an output signal according to a temperature, and more particularly, to a temperature detecting circuit that controls the intensity of a writing laser beam according to the environmental temperature in an MD (Mini Disk) system.
2. Related Art
Conventionally, in a Mini Disk system, a magnetic head is provided to generate a magnetic field in a predetermined part of an MD, and an optical pick-up is provided that emits a laser beam toward this predetermined part of the MD. During recording, the laser beam irradiates the opto-magnetic disk as it rotates, causing the temperature of the irradiated part to rise, and when the temperature reaches the Curie temperature, the coercive force of that part disappears. A magnetic field corresponding to data to be recorded is then applied to the opto-magnetic disk by a magnetic head so that the disk becomes magnetized according to the field. Subsequently, the aforesaid part of the disk is no longer irradiated by the laser beam, and the temperature of that part falls so that the magnetism remains. By repeating this process, data is recorded on the opto-magnetic disk.
In this type of system, a circuit shown in FIG. 3 for detecting environmental temperature is used in generating an intense laser beam that is stable to changes of environmental temperature. In FIG. 3, if the current flowing through a diode 1 is effectively constant with respect to temperature, the temperature characteristic of the forward voltage of the diode 1 is given by: ##EQU1## where:
V.sub.D is the voltage across the two terminals of the diode,
T is the absolute temperature [K]
V.sub.Eg is the energy gap voltage of a semiconductor,
a is a constant related to the temperature characteristic of the degree of mobility
q is an electrostatic charge, and
k is the Boltzmann constant,
From the above: ##EQU2## The current I.sub.1 flowing through a resistor 2 is therefore given by: ##EQU3## where R.sub.1 is the value of the resistor 2. On the other hand, the current I.sub.2 flowing through a resistor 3 is given by: [4] EQU I.sub.2 =V.sub.cc /2.multidot.R.sub.2 3
where R.sub.2 is the value of the resistor 3.
From (1), (2) and (3), the current I.sub.3 flowing through a resistor 4 is therefore given by: ##EQU4##
The control voltage V.sub.C of a VCA(voltage controlled amplifier) 5 is therefore obtained from the voltage decrease of the resistor 4, and ##EQU5##
When the environmental temperature of the MD system increases, therefore, the control voltage V.sub.C decreases and the gain of the VCA 5 falls, so the intensity of the laser beam irradiating the MD becomes weaker. Conversely, when the environmental temperature decreases, the control voltage V.sub.C increases, so the intensity of the laser beam becomes stronger.
However in the circuit of FIG. 3, as is clear from Equation (4), the output signal of the amplifier 6 which is the output signal of the temperature detecting circuit varies according to the temperature T and power supply voltage V.sub.CC. If such a temperature detecting circuit is used to control the intensity of the laser beam in an MD system, therefore, the control voltage V.sub.C varies with the power supply voltage V.sub.CC in addition to the environmental temperature T, so accurate recording on the MD cannot be performed.